Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. An HEMT is a transistor with a junction between two materials having different band gaps, such as GaN and AlGaN.
HEMTs are viewed as an attractive candidate for power transistor applications, i.e., applications in which switching of substantially large voltages and/or currents is required. HEMTs offer high conduction and low resistive losses in comparison to conventional silicon based devices.
HEMTs are commonly formed from III-V semiconductor materials, such as GaN, GaAs, InGaN, AlGaN, etc. In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises at the interface between the AlGaN barrier layer and the GaN buffer layer. The 2DEG forms the channel of the device instead of a doped region, which forms the channel in a conventional MOSFET device. Similar principles may be utilized to select buffer and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. A 2DEG or a 2DHG is generally referred to as a two-dimensional carrier gas. Without further measures, the heterojunction configuration leads to a self-conducting, i.e., normally-on, transistor. Normally-off structures are also possible. In these cases, measures must be taken to prevent the channel region of an HEMT from being in a conductive state in the absence of a positive gate voltage.
One application of type III-V semiconductor technology is a bidirectional switch. A bidirectional switch is a device that is capable of switching voltages of positive or negative polarity. That is, a bidirectional switch is configured to control current flow in both directions. A dual gate type III-V semiconductor bidirectional switch can be realized by providing two HEMT gate structures in series between two electrically conductive terminals that are in contact with the two-dimensional carrier gas. The two HEMTs can share the same drift region (the resistive voltage sustaining part) of the device which means the on-state resistance can be approximately half of a conventional back to back device.
One problem associated with bidirectional switches relates to capacitive coupling between the channel of the device and the underlying semiconductor substrate. In conventional unidirectional semiconductor switching devices, the underlying semiconductor substrate is typically tied to the reference potential terminal (e.g., the source terminal) of the device by substrate contacts. By tying the substrate to a fixed potential, the problem of capacitive coupling between a floating substrate and the channel is eliminated and hence the reliability and stability of the device operation is improved. The same benefit cannot be obtained using a simple electrical contact in the case of a bidirectional switch because there is not a single terminal that is maintained at a reference potential in all states of operations; the voltage polarity across the device changes. Known solutions to this problem suffer from various drawbacks.